1. Field of the Invention
This invention relates to sublimating and cracking apparatus. The apparatus contemplated by this invention can be used to produce a collimated beam of molecules or atoms from a solid source. Such a collimated beam is particularly useful as a source material in the field of molecular beam epitaxy ("MBE").
2. Description of the Related Art
MBE technology involves growing crystalline compounds on a substrate by directing beams of atoms or molecules onto the substrate in a vacuum environment.
Various devices have been developed to produce such atomic or molecular beams useful as a source material in MBE. Such devices typically include a furnace for sublimating a solid source to the gaseous phase (i.e. a "sublimator"). Some devices additionally include a second furnace section for cracking the sublimated molecules into smaller molecular species or to the atomic species (i.e. a "cracker"). For example, a solid tetrameric arsenic (As.sub.4) source may be sublimated to tetrameric As.sub.4 molecules, and then cracked to the trimeric, dimeric, or atomic species (i.e. As.sub.3, As.sub.2, or As, respectively).
One such device known to the Applicants uses a stainless steel sublimation section which is externally heated and a molybdenum tube cracking section which is heated along approximately two inches of its length. The sublimation and cracking sections are aligned on axis in relation to each other, with the discharge end of the sublimation section being restricted by a stainless steel aperture. It has been found that this device often produces less than acceptable levels of purity in the product molecular beam, at least partially due to the typically high temperatures required for sublimation tending to drive impurities out of the stainless steel surfaces into the source material, resulting in contamination.
Another known device uses a sublimation section in the form of an internally heated crucible (typically made of graphite or pyrolylic boron nitride) containing source material. A cracking section is provided using a construction having baffles to enhance cracking. The baffles are typically constructed of tantalum, molybdenum, or other refractory material. However, it has been found that these baffles tend to plug. Furthermore, the baffles tend to require high operating temperatures, and therefore high power inputs, to efficiently function.
Arsenic has proven to be a very useful source material in MBE technology for the growth of such semiconductor compounds as gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs) and indium gallium arsenide (InGaAs). Dimeric arsenic has been shown to have advantages over tetrameric arsenic in the growth of these compound semiconductors. For example, the sticking coefficient for arsenic increases from near zero for As.sub.4 to 0.5 for As.sub.2. This permits a lower arsenic to gallium flux ratio for arsenic stable growth of GaAs, thereby reducing consumption of source material. Furthermore, dimeric arsenic source material tends to reduce required background arsenic pressures in the MBE growth chamber, improve optical and electrical quality of the GaAs and AlGaAs products, and provide better morphology of AlGaAs layers.
Cracking cells have typically been fabricated from graphite, tantalum, molybdenum and pyrolytic boron nitride ("pBN"). However, the outgassing of graphite at high temperatures can result in an unintentional contamination of the epitaxial layers in the growth chamber. Tantalum and molybdenum are relatively easy to design and machine, but it has been suggested that tantalum might react with arsenic, as it does with phosphine. The use of a non-refractory material such as pBN is compatible with standard effusion cell technology, but the resulting inefficient cracking typically requires the use of refractory cracking baffles made from tantalum, molybdenum, rhenium or platinum. These baffles have a tendency to clog and extended operation of the cracker cell with high cracking efficiency often requires variable, high input power. Furthermore, low background carrier concentrations have been difficult to achieve with high temperature cracking cells.
Thus, it is an object of this invention to provide sublimating and cracking apparatus which addresses the disadvantages experienced by the above-described devices.